Fluororesins are excellent in such characteristics as chemical resistance, nonstickiness, gas barrier properties, elution resistance, antifouling properties and bacteria resistance and, therefore, are used for the usage of liquid chemical-transport tubes, tubes for feeding coatings, tubes for drinks and other tubes for industrial use. Since, however, they are expensive, laminated tubes comprising a fluororesin layer and a layer of one of various thermoplastic resins which covers the outer peripheral surface of the fluororesin layer have been proposed.
Laminated resin moldings resulting from lamination of a polyamide resin layer with a fluororesin layer are said to be suited for use as multilayer molded articles required to have good mechanical characteristics and low permeability for liquid chemicals, such as solvents and fuels, which cause deterioration of the polyamide resin, for example, automobile fuel piping tubes or hoses.
As regards multilayer tubes the outer layer of which is made of a polyamide resin, with the inner layer being made of a fluororesin, it has been proposed that the tubes be irradiated to introduce crosslinked structures among molecules of both layers and thereby secure the adhesion between the polyamide resin layer and fluororesin layer (cf. e.g. Patent Document 1). For practicing this technology, however, it is indispensable to introduce expensive equipment and, in addition, the procedural steps become complicated; this is a great problem from the productivity viewpoint.
On the idea that fluororesins themselves should be improved to enable the employment of the coextrusion technique excellent in productivity, various fluororesin materials have been proposed. As one of them, a carbonate and/or haloformyl group-containing, fluorine-containing ethylenic polymer has been disclosed as a fluororesin for use in producing laminates together with a polyamide resin (cf. e.g. Patent Document 2).
However, for use as liquid chemical-transport tubes, tubes for feeding coatings, tubes for drinks or other tubes for industrial use or certain fuel tubes, such resin laminates made of a polyamide resin layer and a fluororesin layer are unsatisfactory in flexibility and, since the polyamide resin is a crystalline resin, the problem of low transparency is encountered in the fields of application where the visibility from the outside is required.
On the other hand, thermoplastic elastomers such as polyurethane-based elastomers have characteristics similar to those of vulcanized rubbers and at the same time can be molded in the same manner as ordinary thermoplastic resins and, in addition, they are excellent in such characteristics as flexibility and transparency; hence, they are used in a wide range of applications, typically in tubes for industrial use. However, there is a problem, namely thermoplastic elastomers are inferior in chemical resistance, chemical liquid impermeability, and bacteria resistance, among others.
To solve the problems with thermoplastic elastomers, resin laminates have been proposed which comprise a layer of a fluororesin excellent in chemical resistance, liquid chemical impermeability and bacteria resistance as well as in nonstickiness, elution resistance and antifouling properties, among others, and a layer of one of various thermoplastic resins as laid on the outer surface of the fluororesin layer.
Thus, a tube resulting from lamination by coextrusion using poly(vinylidene fluoride), for instance, as the fluororesin has been disclosed as the laminate composed of a polyurethane-based elastomer layer and a fluororesin layer (cf. e.g. Patent Document 3 and Patent Document 4). However, the tube obtained is insufficient in interlaminar bonding between the polyurethane-based elastomer layer and fluororesin layer, hence readily undergoes delamination upon repeated bending and vibration during use and, in addition, it has a problem in that it is insufficient in liquid chemical impermeability and chemical resistance.
In cases where a thermoplastic elastomer layer and a fluororesin layer are subjected to lamination with a polyamide resin layer as an intermediate layer, the polyamide resin, when heated and melted in the step of molding, generally tends to be decomposed with ease to generate low-molecular-weight substances or to gelate. To avoid this and simultaneously prevent the discoloration due to oxidation etc., a monocarboxylic acid or a derivative thereof is generally added in the step of polymerization to effect the so-called terminal blocking. Therefore, the polyamide resins in wide use generally have an amine value of lower than 10 (equivalents/106 g).
When the polyamide resin has an amine value of lower than 10 (equivalents/106 g), it is necessary to carry out the coextrusion with the fluororesin at a high temperature of at least 260° C. to attain a sufficient level of interlaminar adhesive strength between the polyamide resin layer and fluororesin layer. At such a high temperature, however, the thermoplastic elastomer causes troubles in the molding step, for example foaming, so that the art has the problem that the polyamide resin and fluororesin cannot be coextruded.
As regards the resin laminate comprising a polyamide resin layer as the outer layer and a fluorine-containing ethylenic polymer layer as the inner layer, it is described in the art that a rubber layer may be provided as the outermost layer (cf. e.g. Patent Document 5). This description, however, only gives a rubber layer as the layer usable as the outermost layer, without giving any concrete description about the rubber species, the laminating conditions or the effects of the rubber layer provided, etc.
A resin laminate consisting of a fluorine-containing ethylenic polymer layer, a polyamide resin layer and a thermoplastic elastomer layer as laminated in that order has been disclosed (cf. e.g. Patent Document 6). This patent document, however, gives no description about such a specific feature of the polyamide resin as the amine value thereof.
Thus, in the art, there is no technology available of coextruding a thermoplastic elastomer, a polyamide resin and a fluororesin, and no resin laminates produced by lamination of a fluororesin with a thermoplastic elastomer are excellent in interlaminar bonding, chemical resistance or transparency.
There are further problems; when such resin laminates produced by lamination of a polyamide resin layer and a fluororesin layer are used under such severe conditions that each face of the laminates is in contact with a liquid chemical, the polyamide resin may be deteriorated as a result of penetration of the liquid chemical from the polyamide resin side, rendering the laminates no more fully durable in practical use, or the laminates may change markedly in size as a result of swelling of the polyamide resin and/or elution of the additive(s) contained in the laminates. Further, the use of a fluororesin in the form of a single layer produces not only the expensiveness problem mentioned above but also the problem that the strength is insufficient, which is a weak point of the fluororesin.
When such resin laminates consisting of a polyamide resin layer and a fluororesin layer laminated with each other are used as underground tubes in gas stations, in particular, they are generally used in the form of double tubes and, in that case, it is likely for the inner tube to come into contact with a fuel both on the outer side and on the inner side, and the outer tube is of course buried in the ground. Therefore, both tubes are required to have low fuel permeability and sufficient tube strength for embedment while retaining such characteristics as chemical resistance, nonstickiness, elution resistance, antifouling properties and bacteria resistance on both sides. However, there is no one available that satisfies all of such requirements.    Patent Document 1: Japanese Kokai Publication H05-8353    Patent Document 2: International Laid-open Patent Application WO 99/45044    Patent Document 3: Japanese Kokai Publication H08-142151    Patent Document 4: Japanese Kokai Publication H10-286897    Patent Document 5: International Laid-open Patent Application WO 01/58686    Patent Document 6: International Laid-open Patent Application WO 98/58973